1. Field of the Invention
The present invention relates generally to a device and method for preventing the undesired passage of emboli from a venous blood pool to an arterial blood pool. The invention relates especially to a device and method for treating certain cardiac defects, especially patent foramen ovales and other septal defects through the use of an embolic filtering device capable of instantaneously deterring the passage of emboli from the moment of implantation.
2. Description of Related Art
The fetal circulation is vastly different than the normal adult circulation. The blood circulating in a fetus is oxygenated by the placenta, not the developing lungs. Therefore, the fetal circulation directs only a small percentage of the circulating blood to the fetal lungs. Most of the circulating blood is shunted away from the lungs to the peripheral tissues through specialized vessels and foramens that are open (“patent” during fetal life. In most people these specialized structures quickly close after birth. Unfortunately, they sometimes fail to close and create hemodynamic problems that can be fatal if left untreated.
A diagram showing the blood circulation of a human fetus is illustrated in FIG. 1. The umbilical arteries branch off of the iliac arteries and deliver unoxygenated blood to the placenta. The fetal blood travels through the capillary bed in the placenta and transfers carbon dioxide to the maternal blood and takes oxygen and other nutrients from the maternal blood. The umbilical vein returns oxygenated blood to the fetus. Most of the oxygenated blood from the umbilical vein bypasses the developing liver and travels through a specialized vessel called the ductus venosus to the inferior vena cava and then into the right atrium. A good portion of the oxygenated blood from the inferior vena cava is directed across the right atrium and into the left atrium through a specialized curtain like opening in the heart called the foramen ovale. The blood from the left atrium then enters the left ventricle and then into the aorta where it travels to the head and other body tissues delivering the needed oxygen and nutrients.
The small amount of blood entering the right atrium that does not pass through the foramen ovale, most of which comes from the superior vena cava, flows into the right ventricle and then gets pumped into the pulmonary trunk and pulmonary arteries. Some of this blood is pumped into the developing lungs. However, the fetal lungs are collapsed which causes a high resistance to blood flow. Another specialized vessel, called the ductus arteriosus, is a vessel that connects the high pressure pulmonary artery to the lower pressure aorta. Therefore, most of the blood in the pulmonary artery flows into the lower pressure aorta through this specialized vessel.
Upon birth, the circulatory system goes through profound changes. The flow through the umbilical arteries and umbilical vein stops and consequently the flow through the musculature around the ductus venosus constricts and the blood flow through the ductus venosus stops. The lungs fill with air and the resistance to blood flow into the lungs drastically decreases. The corresponding pressure in the right atrium, right ventricle, and pulmonary arteries also decrease. The decrease in pressure in the right atrium causes the curtain like opening of the foramen ovale to close, driving more blood into the right ventricle and then to the lungs for oxygenation. Over time, the foramen ovale is replaced with a membrane called the fossa ovalis. Similarly, the decrease in pressure in the pulmonary arteries reduced the pulmonary arterial pressure to the same as or slightly less than the pressure in the aorta, which stops or reverses the flow through the ductus arteriosus. Once the muscular tissue of the ductus arteriosus is perfused with well oxygenated blood, the muscle begins to constrict and close the ductus arteriosus. The ductus arteriosus normally closes within about one week of life.
Usually over time, the unique openings of the fetal circulation become obliterated and a solid mass of tissue forms where these opening once were. However, in some people the opening remain. A patent ductus venosus after birth is very rare and almost always fatal. A patent ductus arteriosus occurs in about 1 out of every 5000 births. The patent ductus arteriosus once diagnosed is either medically treated or surgically ligated to close the ductus. In about one of four people, the foramen ovale does not seal shut, instead it remains patent. Such defects usually measure 10 mm or more in diameter and occupy one third or more of the length of the atrial septum in echocardiographic four chamber sections. Since the pressure in the left atrium is about two to four mm Hg greater than the pressure in the right atrium, the curtain like opening usually remains shut. However, if the pressure in the right atrium increases, such as upon heavy lifting or while performing a Valsalva type maneuver, the curtain like fold of tissue opens and the blood flows from the right atrium to the left atrium.
Studies have shown that adults with strokes of unknown origin, i.e., cryptogenic strokes, have about twice the normal rate of patent foramen ovales than the normal population. Although there is a correlation between strokes and patent foramen ovales, it is currently unknown why this correlation exists. It is theorized that blood clots and plaque that have formed in the peripheral venous circulation (in the legs for example) break off and travel to the heart. Normally, the clots and plaque get delivered to the lungs where it is trapped and usually cause no harm to the patient. Patients with a patent foramen ovale, however, have a potential opening that the clots or plaque can pass through the venous circulation and into the arterial circulation and then into the brain or other tissues to cause a thromboembolic event like a stroke. The clots may pass to the arterial side when there is an increase in the pressure in the right atrium. Then the clots travel through the left side of the heart, to the aorta, and then to the brain via the carotid arteries where they cause a stroke and the associated neurological deficits.
A number of atrial septal defects (ASD) closure devices have been developed and investigated in an attempt to develop a nonsurgical, transvenous method of occlusion of ASD. These include the Sideris Buttoned device, the Angel Wing Das device, the atrial septum defect occlusion system (ASDOS) device, the Amplatz Septal Occluder, the CardioSEAL/StarFlex devices, and the Gore/Helix devices. Unfortunately, each of these devices have distinct disadvantages and limitations ranging from the size of the device delivery sheath, ease of implantation, feasibility, safety and effectiveness. The Sideris buttoned device is made of a polyurethane foam occluder with a Teflon coated wire skeleton, which is positioned within the left atrium, and a polyurethane foam rhomboid shaped counteroccluder with a Teflon coated wire skeleton, which is positioned in the right atrium. The major disadvantage with this device is the lack of a centering mechanism. For this reason, use of the devices at least two times the size of the stretched ASD is required. (Sievert H. Koppeler P. Rux S: Percutaneous closure of 176 interarterial defects in adults with different occlusion devices—6 years of experience [abstract], J. Am. Coll. Cardiol 1999, 33:519A.) Consequently, closure of defects may become difficult because the required size may be too large for the atrial septum to accommodate, or the device may impinge critical structures. There are also reports that the retrieval of the Sideris button device after incorrect deployment is difficult. (See, e.g., Rigby, Michael L., The Era of Transcatheter Closure of Atrial Septal Defects, Heart; 81:227-228 (1999)).
The “Angel Wings” device comprises two square frames made of superelastic Nitinol wire, each square frame having four legs that are interconnected by flexible islets at the corners. The wire frames are covered by polyester fibers. There is a conjoint suture ring of the right and atrial discs, which allow self centering on deployment. The device is delivered through an 11-13 F Mullins sheath. The major disadvantage of using this device is the attendant risk of aortic perforation cause by its sharp eyelet corners. In fact, the Angel Wings device was withdrawn from further clinical trials because of this problem. (Syamaxundar Rao, P., M.D., Summary and Comparison of Atrial Septal Defect Closure Devices, Current Interventional Cardiology Reports 2000, 2:367-376 (2000)). The device is also ill-suited for treating fenestrated defects.
The atrial septal defect occlusion system (ASDOS) prosthesis (Microvena Corp., White Bear Lake, Minn.) consists of two umbrellas made of Nitinol and a patch or porous polyurethane attached to the left and right atrial devices. The device is introduced transvenously over a long veno-arterial guidewire and through an 11 F venous transeptal sheath. While the device is retrievable in the event of malpositioning before release of the device, it requires a complex procedure to implant, and the components are known to have high incidences of thrombosis thrombosis. It is also reported that frame fractures have been detected in 20% of the patients treated with this device.
The Amplatzer device is the subject of U.S. Pat. No. 5,944,738 to Amplatzer, et al. This device is a saucer-shaped device formed from a mesh of fine Nitinol wires with a central connecting cylinder having a diameter similar to that of the stretched diameter of the defect. Thrombosis following implantation of the device is induced by three polyester patches. The device is delivered through a 6-10 F Mullins sheath. The primary disadvantage with this device is that it is ill-suited for closing fenestrated defects. Moreover, the device is a thick, bulky profile which dramatically increases the chances that the device will interfere with the heart's operation. Another disadvantage is its known capacity for incomplete endothelialisation with thrombus formation.
The CardioSEAL® device (NMT Medical, Inc.) is the subject of U.S. Pat. No. 6,206,907 to Marino, et al. This occlusion device is comprised of a center section to which stranded wire elastic shape memory fixation devices are attached. The fixation devices hold the occlusion devices in place once it is inserted into an aperture. Attached to the fixation devices are polyvinyl foam sheets which occlude the aperture. While the CardioSEAL is deemed to be relative easy to use, it is reported that, of all the devices, the CardioSEAL device has the highest incidence of arm fractures, which has raised serious issues concerning its safety. Moreover, the CardioSEAL device, like the Amplatzer device is relatively large, and requiring at least a lO F or ll F delivery systems, and an undue amount of hardware within the heart. These characteristics increase the chance that the device will interfere with the heart's operation, lend to residual shunting and/or embolization. The size of the CardioSEAL device also renders it less suitable for small children.
The STARflex® device (NMT Medical, Inc.) is an updated version of the CardioSEAL device, which includes a self-centering mechanism consisting of four flexible springs which pass between the two fabric disks. While this added feature may reduce the instances of residual shunting, the aforementioned defects and disadvantages of the CardioSEAL are still a concern.
In view of these drawbacks and related-risks, the method of choice to close a patent foramen ovale is still open heart surgery and ligation of the foramen ovale to close it. Surgery, however, is obviously associated with the usually risks of general anesthesia, open heart procedures, infections, etc. Thus, there is a need for a safe, cost-effective, and easily implantable device and method for preventing the passage of emboli from an arterial blood pool and a venous blood pool which is not subject to the defects and disadvantages of known devices.